If dir_spec specifies a path that includes directories, which
    do not exist, intermediate directories are also created.
    This differs from the behavior of the UNIX system where these
    intermediate directories must exist and will not be created.

    If you do not specify any optional arguments, the C RTL
    gives the directory your UIC and the default version limit of the
    parent directory, and arbitrarily places the directory within the
    volume set. You cannot get the default behavior for the uic or
    max_versions arguments if you specify any arguments after them.

                                   NOTE

       The way to create files with OpenVMS RMS default protections
       using the UNIX system-call functions umask, mkdir, creat,
       and open is to call mkdir, creat, and open with a file-
       protection mode argument of 0777 in a program that never
       specifically calls umask. These default protections include
       correctly establishing protections based on ACLs, previous
       versions of files, and so on.

       In programs that do vfork/exec calls, the new process image
       inherits whether umask has ever been called or not from
       the calling process image. The umask setting and whether
       the umask function has ever been called are both inherited
       attributes.

    The file protection supplied by the mode argument is modified by
    the process's file protection mask in such a way that the file
    protection for the new directory is set to the bitwise AND of the
    mode argument and the complement of the file protection mask.

    Default file protections are supplied to the new directory
    from the parent-directory such that if a protection value bit
    in the new directory is zero, then the value of this bit is
    inherited from the parent directory. However, bits in the parent
    directory's file protection that indicate delete access do not
    cause corresponding bits to be set in the new directory's file
    protection.